The present invention relates to the dispensing of viscous materials, and more particularly, to the rapid dispensing of minute amounts of adhesives in mounting electronic components on circuit boards prior to solder reflow.
In the manufacture of circuit boards it is frequently necessary to apply small amounts of viscous materials, i.e those with a viscosity greater than fifty centipoise. Such materials include adhesives, solder paste, solder flux, solder mask, grease, oil, encapsulants, potting compounds, inks, silicones, RTV and cyanoacrylates. Heretofore the common methods of application have included screening, pin transfer and dispensing from a syringe or valve. Screening requires a template and is not readily adaptable to changing application patterns. Pin transfer is relatively fast but the tooling is intensive and inflexible. Syringe dispensing is widely used and is accomplished with pneumatic mechanisms or positive displacement valves. It is difficult to dispense more than four dots of viscous material per second with a syringe dispenser. It is critical to such dispensers that the height of the needle or other dispensing tip be carefully adjusted relative to the work piece or else the blob of viscous material will be misformed or will not break away from the needle tip. Thus complex height sensing mechanisms are required as shown in U.S. Pat. No. 4,661,368 of Rhode et al., U.S. Pat. No. 4,762,578 of Burgin, Jr. et al., U.S. Pat. No. 4,967,933 of Maiorca et al. and U.S. Pat. No. 5,110,615 of Maiorca et al.
Syringe dispensers usually place the tip very close to the substrate, i.e. 0.005 inches for a very small dot and 0.060 inches for a large dot. The viscous material is pushed out of the tip and contacts the substrate while it is still connected to the tip. If the viscous material fails to contact the substrate, it will not adhere to the substrate and a poorly formed dot will result. The contacting with the substrate is called "wetting." After the viscous material contacts the surface of the substrate, the tip is pulled back and the resulting string is broken to form a dot.
Prior viscous material dispensers have typically included a reservoir, feedpath, pinch points, gates or check valves, a chamber and an exit port. These can prevent rapid flow of viscous materials. They can also become clogged.
It would therefore be desirable to provide a method for rapidly dispensing precisely formed minute quantities of adhesive and other viscous materials, particularly in connection with the attachment of surface mounted electronic components to PC boards prior to solder reflow. This would enable variable patterns of adhesive, such as lines of dots, to be rapidly laid down before placement of the components thereon. It would be further desirable to provide a method for rapidly dispensing minute quantities of adhesive and other viscous materials, particularly in connection with the attachment of surface mounted electronic components to PC boards, that would be insensitive to height variations in the workpiece, such as those attributable to warpage in the PC boards.